The Loss of Mains (LOM) protection is often required when distributed generation (DG) is connected to a power system network and the DG generator is not allowed to stay connected to the network if the main supplying network is disconnected. This situation is also called an unwanted islanding operation.
In normal operation, when the distributed generation generator is connected to the power system network, there is a phase difference between an electromagnetic force and terminal voltages of the generator. This is illustrated in FIG. 1, where the power system network/main feeder network is depicted by reference number 100 and the distributed generator system/network is depicted by reference number 102. Em is the electromagnetic force, Xd is the reactance, and Igenerator is the current of the distributed generation generator. Z is the load impedance in the network, and Vm is the terminal voltage of the generator. The voltage drop ΔV between Em and Vm can be calculated as (1):ΔV=Xd*Igenerator  (1)
The vector representation on the right shows how the voltage drop ΔV depends on Vm and Em.
FIG. 2 shows a fault situation, where the supplying network 100 is disconnected by opening a circuit breaker at the substation switchgear, for instance, and the DG generator 102 stays connected to the islanded network. As a consequence of an increasing current Igenerator, as shown by the vector representation on the right, the voltage drop gets a new value. When this occurs, the terminal voltage Vm of the DG generator 102 will suddenly shift to a new value.
In a known method, the detection of the change of the terminal voltage Vm has been carried out by measuring time intervals of successive half cycles of the measured sinusoidal voltage signal, which is illustrated by FIG. 3. In that method, the calculation of the half cycle π interval starts and ends from/to zero crossing detection. In FIG. 3, there is a fault in the network occurring in the third half-cycle, whereby the zero-crossing of the signal occurs at moment π+Δθ instead of the principal moment π.
Signal harmonics and other measurement interferences can easily cause maloperation to the measurement. Another problem is oscillation in the vicinity of the zero crossing which has, in practice, required a hysteresis handling of the signal. Zero crossing detection can even be prevented by a direct current component present in the signal.
WO 2011/038756 refers to “Novel protection methods for active distribution networks with high penetrations of distributed generation, Year II report authored by A. Dysko, G. Burt and R. Bugdal, June 2006”. It presents a frequency domain based “method 1” with locally delayed voltage angle signals. In the method, first harmonics recent and arbitrarily delayed historical fft-phasor angles are subtracted from each other, and if the subtraction exceeds a threshold value, the protection function will be applied. The method presented in WO 2011/038756 has the drawback in that an indication of a fault situation in the network is slow. This leaves less time for reacting to network changes, which may lead to material damages or cause a security hazard, for instance.
There is thus a desire for an improved loss of mains detection in an electric network.